This invention relates to a method and apparatus for hardening the heads of railway rails to enhance their wear resistance. As the technical literature summarized in U.S. Pat. No. 4,611,789 indicates, the best combination of wear hardness and certain other mechanical properties in rail heads are achieved by a fine pearlite microstructure. As this patent also indicates, such a microstructure may be developed by heating the heads of an axially moving rail to austenizing temperatures and then quenching the heads with air or other cooling means but not so rapidly as to develop hard martensite or bainite in the microstructure.
Until recently, rails having a nominal minimum head hardness of 248 Brinell was the standard rail in the industry. The chemistry of these rails was such that rails could be quenched with little risk of developing significant amounts of martensite. As loads and speeds have increased however, the industry has specified more wear resistant rails. Thus, rails having a nominal head hardness of 300 Brinell are now specified as standard. The chemistry of 300 BHN rails is such that the quenching operation must be very carefully controlled to avoid overquenching the head to form martensite. In addition, the hardness distribution pattern developed in the rail head must meet industry standards. Thus, the surface hardness of the rail head must be between 346 and 393 BHN, with 388 being desired; the 321 BHN hardness line must be at least 9/16" from the shoulder of the head and 1/2 from top center; and the hardness must decrease uniformly from the surface to the interior.